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NGC 891, an edge-on spiral galaxy well known to amateur deep-sky observers, contains enough molecular hydrogen to account for all the “missing mass” in a typical spiral. This high-resolution image was taken with the WIYN Telescope on Kitt Peak in Arizona.

Article from Sky &Telescope January 2000 page 20.

SEVERAL KINDS OF UNSEEN “Dark Matter” are either known or suspected to exist throughout the universe. One kind of invisible matter adds unseen mass to individual galaxies, above and beyond a galaxy’s visible stars and interstellar matter swarms of brown or white dwarfs, yet-to-be-discovered atomic particles called WIMPs or axions, and hypothetical “quark nuggets” have been proposed to account for it.

But the dark matter in galaxies may not be so exotic or even very dark. According to two Dutch astronomers, most or all of it may be ordinary molecular hydrogen (H2), which, unlike atomic hydrogen (H), is invisible except at certain infrared wavelengths.

Using the European Space Agency’s Infrared Space Observatory (ISO), Edwin A.Valentijn (Kapteyn Institute, Groningen) and Paul P. van der Werf (Leiden Observatory) detected huge amounts of relatively warm molecular hydrogen in NGC 891, an edge-on galaxy 30 million light-years away in Andromeda. In the September 1, 1999, Astrophysical Journal Letters they claim that their result “matches well the mass required to resolve the problem of the missing matter of spiral galaxies.”

Molecular hydrogen is notoriously difficult to observe. However, the two lowest rotational energy states of this molecule produce weak spectral lines at the far-infrared wavelengths of 28.2188 and 17.0348 microns, a spectral region covered by ISO’s Short Wavelength Spectrometer. A few years ago Valentijn reported the first extragalactic detection of these lines in the center of NGC 6946. Now, the study of NGC 891 reveals that molecular hydrogen is all over the place. Valentijn and van der Werf conclude that the galaxy contains 5 to 15 times more molecular than atomic hydrogen (which is easily observed using radio telescopes). They write, “It is well established that if there is about 10 times as much molecular hydrogen as atomic hydrogen in the disks of spiral galaxies, then the missing mass problem [in galaxies] is solved.”

Since NGC 891 is a run-of-the-mill spiral, it is reasonable to assume that other galaxies may harbor similar amounts of molecular hydrogen. But this may be hard to confirm. The current observations are right at the sensitivity limit of ISO’s spectrometer. Moreover, the gas in NGC 891 is relatively warm (80º to 90º K) with still warmer patches (150º to 230º K), which makes it easier to spot. A thin background of molecular hydrogen would be much harder if not impossible to detect in our own Milky Way because the faint signal would be smeared across the whole sky.

The gas that Valentijn and van der Werf have detected resides in the galaxy’s flat disk. What about the dark matter supposedly in galaxy halos?

Surprisingly, the authors claim that none may be necessary. “Our results give a much stronger footing for the ‘ordinary matter’ simple solution of the dark matter problem, in the form of massive clouds in the disks of galaxies:” they say. According to Valentijn, the “halo culture” that has grown up around the dark-matter problem might never have arisen if the ISO results had been known earlier. Nevertheless, “the problem is complex enough to avoid drawing quick conclusions:,” he says. For instance, little is known about the warming mechanism for such huge amounts of gas.